Default help needed

[Rumblee1]

I've got an axon7 and am thrilled with it. One thing I'm having a problem with. Is there any way to set download over wifi only as a download default. I downloaded 2 movies, and in spite of being connected to my home wifi with strong signal, it downloaded the movies over data. I suppose I could shut down the data, but I shouldn't have to do that. I have a default on my S7 edge.